1. Field of the Invention
The invention relates to a variable compression ratio internal combustion engine capable of changing the compression ratio that is the ratio of the maximum value to the minimum value of the volume of a combustion chamber that changes with the movement of the piston.
2. Description of the Related Art
Variable compression ratio internal combustion engines have been proposed which change the compression ratio by moving the cylinder block relative to the crankcase in the direction of an axis (center axis) of a cylinder bore (hereinafter, simply referred to also as “up-down direction”). For example, one of the variable compression ratio internal combustion engines has a cylinder block that is disposed so as to be relatively movable in the up-down direction with respect to a crankcase, and a variable compression ratio mechanism.
In the cylinder block, in-line arranged four cylinder bores are formed. A piston is housed in each cylinder bore. The pistons are linked to a crankshaft. The crankshaft is rotatably supported by the crankcase. Furthermore, the variable compression ratio internal combustion engine includes a cylinder head. The cylinder head is fixed to a top portion of the cylinder block.
The variable compression ratio mechanism includes a block-side bearing-forming portion, a case-side bearing-forming portion and a shaft-shaped drive portion.
The block-side bearing-forming portion is fixed to an outer wall surface (side wall surface) of the cylinder block so as to extend out from the outer wall surface in a region that contains an crankcase-side end portion of the outer wall surface (a lower end portion of the cylinder block).
The case-side bearing-forming portion is made up of an upstanding wall portion and a cap portion. The cap portion is fixed to the upstanding wall portion that is formed on an upper portion of the crankcase.
The shaft-shaped drive portion includes a plurality of eccentric cam portions, and is disposed so as to extend through a cylindrical bearing hole formed in the block-side bearing-forming portion, and a cylindrical bearing hole formed in the case-side bearing-forming portion. Then, the shaft-shaped drive portion is rotated about a predetermined axis by a driving device. At this time, the shaft-shaped drive portion rotates in contact with the surfaces that define the cylindrical bearing holes formed in the block-side bearing-forming portion and the case-side bearing-forming portion, and causes a shift in the direction of eccentricity. Thus, the cylinder block can be slid relative to the crankcase to the top dead center side. As a result, since the distance between the cylinder block and the crankcase becomes longer, the volume of the combustion chamber when the piston is at the top dead center (the minimum value of the volume of the combustion chamber) becomes larger, and therefore the compression ratio becomes lower. In this manner, according to the foregoing internal combustion engine, the compression ratio can be changed (e.g., see Japanese Patent Application Publication No. 2003-206771 (JP-A-2003-206771)).
When a mixture gas burns in a combustion chamber defined by a wall surface that defines the cylinder bore, a lower surface of the cylinder head, and a top surface of a piston, the pressure of the gas in the combustion chamber becomes very high. Due to this pressure, the lower surface of the cylinder head is pressed upward by the pressure in the combustion chamber, and the top surface of the piston is pressed downward by the same pressure. Therefore, a force in an upward direction is exerted on the cylinder block to which the cylinder head is fixed. On the other hand, a force in a downward direction is exerted on the crankcase that supports the crankshaft linked to the piston. As a result, a crankcase-side portion of the surface that defines the bearing hole of the block-side bearing-forming portion receives a force caused by the shaft-shaped drive portion 53, and is therefore pressed downward.
Since a cylinder head-side end portion of the cylinder block is fixed to the cylinder head as mentioned above, the rigidity of the cylinder head-side end portion of the cylinder block is relatively high. On the other hand, a crankcase-side end portion of the cylinder block has a relatively low rigidity since the portion is not fixed to the crankcase. Furthermore, since the force exerted on the bearing hole of the block-side bearing-forming portion acts at a position that is apart outward from the outer wall surface of the cylinder block to which the block-side bearing-forming portion is fixed, the force acts on the cylinder block as a force that tends to bend a lower end portion of the cylinder block inward (bending moment).
In other words, a pressing force in an inward direction of the cylinder block (pressing direction) is exerted on a region in the outer wall surface of the cylinder block to which the block-side bearing-forming portion is fixed. Due to this pressing force, the wall surface defining the cylinder bore deforms in an inward direction of the cylinder bore. As a result, there is possibility that the friction force between the wall surface defining the cylinder bore and the piston may increase, and the fuel economy may deteriorate, or that the amount of inflow of lubricating oil into the combustion chamber may increase leading to useless consumption of lubricating oil.